[Haskell] Re: Parameterized Show
benjamin.franksen at bessy.de
Fri Nov 19 09:27:08 EST 2004
On Friday 19 November 2004 08:54, Adrian Hey wrote:
> On Tuesday 16 Nov 2004 1:52 pm, Ben Rudiak-Gould wrote:
> > George Russell wrote:
> > > Sorry, but I like implicit parameters, I use them, and I'm not
> > > going to stop using them because beta conversion no longer
> > > preserves semantics.
> > You'll find that many people here don't agree with this view in
> > general (though there's been surprisingly little backlash against
> > implicit parameters in particular).
> I would like to lash against them. I was unaware of the problems you
> describe, but their existence doesn't surprise me. In view of the
> controversy that a certain other perfectly safe, reasonable (and
> necessary) language extension proposal has generated recently the
> ready acceptance of this kind of brokeness is surprising. Frankly,
> the idea that anyone would want to jump through hoops to add them to
> a purely functional language sounds bizarre to me. Safe beta
> conversion really ought to be a sacred cow.
> Still, at least they're not enabled by default. With any luck we
> won't see too many people shooting themselves in the foot because
> they're too lazy to pass their parameters explicitly.
Implicit parameters are evil, agreed. Their deficiencies should be added
as a warning to the docs (with many exclamation marks). But toplevel
things with identity (TWI) are evil as well, *especially* if they are
easy to use. Implicit parameters at least have the advantage that they
are obscure and require changes to function signatures, so that
(hopefully) not many people use them.
The toplevel '<-' bindings proposal encourages bad library and program
design by making TWIs easy and (apparently) safe. It is better to make
people think hard about how to avoid them in the first place. (BTW,
toplevel stdin, stdout, and stderr are evil too.)
I know of exactly one good reason to use TWIs in Haskell. Which is: to
interface C libraries that are broken because of the fact that TWIs are
so easy to create in C.
Introducing TWIs in Haskell is like deliberately spreading a disease
into an area that has avoided it up to now by strict quarantine
measures. Of course these measures are often inconvenient and some
effort is required in order to make communication with the ill populace
possible. And of course every now and then people come along
complaining that everything would be easier if one would just remove
all those decontamination barriers...
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